Jae-Woo Joung

Environmentally friendly synthesis of organic-soluble silver nanoparticles for printed electronics

In this study, we attempted to synthesize organic-soluble silver nanoparticles in the concentrated organic phase with an environmentally friendly method. The fully organic phase system contains silver acetate as a silver precursor, oleic acid as both a medium and a capping molecule, and tin acetate as a reducing agent. Monodisperse silver nanoparticles with average diameters of ca. 5 nm can be easily synthesized at large scale. Only a small usage of tin acetate ( 90%). Also, it was investigated that the residual tin atom does not exist in the synthesized silver nanoparticles. This implied that tin acetate acts as a reducing catalyst.

Characteristics of microstructure and electrical resistivity of inkjet-printed nanoparticle silver films annealed under ambient air

The microstructure and electrical resistivity of inkjet-printed silver (Ag) films annealed under ambient air were characterized. Analyses of the impurity amounts in the films using secondary-ion mass spectrometry showed that the decomposition temperature of the capping molecules was just below 170°C. Both the characteristics of the microstructure and electrical resistivity when annealed at low temperatures (lower than the decomposition temperature) were significantly different from those when annealed at high temperatures. The results show that neither microstructural features, such as grain size, nor the amounts of impurities can explain both the magnitude and characteristic decrease in electrical resistivity. The changes in electrical resistivity can be described using exponential decay kinetics. The corresponding activation energy of 0.44 eV when annealed at the high temperatures is explained by the migration of point defects such as vacancy–oxygen pairs. On the other hand, negligible dependence on temperature was identified when annealed at low temperatures, which was attributed to decomposition of the capping molecules. The results indicate the importance of controlling the defects of nanoparticles and the properties of capping molecules from the viewpoint of electrical optimization of metallization fabricated using inkjet printing.

Effect of microstructure on electrical and mechanical properties: Impurities of inkjet-printed Ag and Cu interconnects

Inkjet printing technology is a pattern-on-demand technology which has numerous advantages. However, this technology needs an additional thermal treatment, i.e., drying process. This treatment results in microstructure evolution which is expected to relate to properties of film. The microstructure, electrical and mechanical properties of the inkjet printed Ag and Cu films were characterized as drying process. Model study on electrical resistivity of Ag film shows that the decomposition of capping molecule plays a key role in microstructure evolution and electrical resistivity. The effect of ambient in thermal treatment of inkjet printed Cu film also investigated in this purport. The adhesion strength as a mechanical property was measured by 4 point bend test using sandwiched structure. Strengthening of adhesion was observed as densification of inkjet printed film.

Improvement of Electrical and Mechanical Properties of Ag Nanoparticulate Films by Controlling the Oxygen Pressure

Improvement of Electrical and Mechanical Properties of Ag Nanoparticulate Films by Controlling the Oxygen Pressure Seol-Min Yi, Ji-Hoon Lee, Na-Rae Kim, Sungil Oh, Seonhee Jang, Donghoon Kim, Jaewoo Joung, and Young-Chang Joo Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea Samsung Electro-Mechanics, Central R&D Institute, Suwon, KyoungGi-Do 443-743, Korea

Design and characterization of piezoelectric inkjet for micro patterning of printed electronics

A design verification system based on multiphysics modeling and micro-electro-mechanical systems (MEMS) fabrication has been established to develop piezoelectric inkjet printheads for micro-patterning on printed electronics. Piezoelectric printheads have been fabricated with silicon and silicon on insulator (SOI) wafers by MEMS fabrication and post-processing package. Transient displacements of a piezoelectric actuator according to voltage waveform are measured by Laser Doppler Vibrometer (LDV), and compared with numerical predictions by the three-dimensional piezoelectric-structure interaction modeling. Key issues in design and fabrication of piezoelectric inkjet printheads are investigated: printhead configuration, input voltage waveform, hydrodynamic and structural crosstalks, acoustic wave propagation (or effect of limited compressibility), and meniscus instability at high frequency. The present design verification system has shown its promising applicability to novel-concept designs of inkjet printheads for wide range of printed electronics and bio-applications.

The Effects of Driving Waveform of Piezoelectric Industrial Inkjet Head for Fine Patterns

This paper presents the effect of driving waveform for piezoelectric bend mode inkjet printhead with optimized mechanical design. Experimental and theoretical studies on the applied driving waveform versus jetting characteristics were performed. The inkjet head has been designed to maximize the droplet velocity, minimize voltage response of the actuator and optimize the firing frequency to eject ink droplet. The head design was carried out by using mechanical simulation. The printhead has been fabricated with Si(100) and SOI wafers by MEMS process and silicon direct bonding method. To investigate how performance of the piezoelectric ceramic actuator influences on droplet diameter and droplet velocity, the method of stroboscopy was used. Also we observed the movement characteristics of PZT actuator with LDV(laser doppler vibrometer) system, oscilloscope and dynamic signal analyzer. Missing nozzles caused by bubbles in chamber were monitored by their resonance frequency. Using the water based ink of viscosity of 4.8 cps and surface tension of 0.025N/m, it is possible to eject stable droplets up to 20kHz, 4.4m/s and above 8pL at the different applied driving waveforms.

Numerical and Experimental Evaluation of Picoliter Inkjet Head for Micropatterning of Printed Electronics

A design process based on multiphysics modeling and micro-electro-mechanical systems (MEMS) fabrication has been established to develop a picoliter inkjet printhead for micro-patterning for printed electronics. Piezoelectric actuator is designed with numerical analysis using Covent-Ware with consideration of electrical characteristic of piezoelectric material and physical characteristic of silicon structure. The displacements of a piezoelectric actuator according to voltage waveform are evaluated and verified by laser doppler vibrometer (LDV). Piezoelectric printheads have been fabricated with silicon and silicon-on-insulator (SOI) wafers by MEMS process and silicon to silicon bonding method. As a preliminary approach, liquid metal jetting phenomena are identified by simulating droplet ejection and droplet formation in a consequent manner. Parametric studies are followed by the design optimization process to deduce key issues to inkjet head performance: printhead configuration, input voltage amplitude, ink viscosity and meniscus movement using computational fluid dynamics (CFD). By adjusting the driving voltage along with optimizing the drive waveform, the droplet volume and velocity can be controlled and evaluated by a drop watcher system. As a result, inkjet printhead capable of ejecting 1 pL droplet, which is required by electronic applications such as fabricating metal lines on printed circuit board (PCB), is developed.

Effect of Sintering Atmosphere on the Microstructure of Gold Nanoparticulate Film

Over the past few decades, metallic nanoparticles (NPs) have been of great interest due to their unique properties which distinguish them from those of bulk metals. Many attempts have been conducted to investigate the characteristics of NPs and their applications. However, the sintering process which converts metallic NPs to conductive film was not established yet. In this study, the microstructure evolution of Au NPs after sintering under different thermal condition was examined and the film quality was studied based on densification, organic residues and electrical resistivity. Au NP ink dispersed in a toluene were spin coated on Ni-plated FCCL or Si substrates and thermally treated in a furnace under different sintering profiles under various types of flows such as air, nitrogen (N 2 ), or reducing atmosphere of formic acid (FA). The Au ink was consisted of Au NPs coated with an organic capping molecule. The capping molecules not only help NPs to disperse but prevent aggregation and precipitation of NPs out of solution. When the NPs are treated by thermal process, the surface ligands from capping molecule start to decompose and necking and melting of NPs occur producing the film with the electrical conductivity. The diameter of Au NPs was approximately between 5-7 nm with spherical shape. The Au film sintered under air showed only necking between neighboring Au NPs without further grain growth. When Au NPs films were sintered under N 2 atmosphere, NPs fused together in clusters. Under sintering with flows of FA, a larger area of pores due to the volume shrinkage of the film was observed since an agglomeration and melting of NPs were considerably progressed compared to the film sintered under N 2 . Sintering with a flow of a single gas such as air or N 2 showed organic residues in the film indicated by C-H or C-O stretch peaks. However, when mixed flows of FA and N 2 were applied, there was no IR peaks from organic substances observed in the film. It is assumed that the organic capping molecules surrounding the Au NPs were removed significantly with sintering with two flows of FA and N 2 . The microstructure showed less pore distribution and lower level of organic residues compared to those sintered under air, N 2 , or FA atmospheres. The electrical resistivity was about twice of bulk value of 2.44 μΩ -cm. Overall Au NPs film sintered under FA and N 2 resulted in a better sintering effect based on densification of the film and level of residual organics, translating into a relatively high electrical conductivity.